As the demand for portable flexible displays and large size flexible displays increases, display materials that can be folded or rolled are needed. For this purpose, an electrode material for display use should be transparent (transmittance of 80% or more), have a low resistance (sheet resistance), and have a high strength so that it is mechanically stable even when bent or folded, and have a thermal expansion coefficient that is similar to a plastic substrate, so that even when a device using the display is overheated or placed under a high temperature, it is not disconnected and its sheet resistance does not change significantly.
However, in the case of a flexible display, in order to realize flexibility, low price, and lightweightness, conventional glass substrates need to be substituted to transparent and flexible plastic substrates. Plastic substrates have an extremely low thermal resistance temperature, and a transparent substrate having a thermal resistance of 150° C. to 200° C. has been developed. Accordingly, when forming a transparent electrode by sputtering an ITO electrode on a plastic substrate, reducing the specific resistance of the film may be restricted, due to difficulty in heating.
Not only that, ITO has a lower coefficient of expansion compared to a high molecular compound, and thus due to a long thermal history when producing or driving a device, the substrate and electrode thermal expands in different ratios, causing deformation of the substrate. Furthermore, a conventional ITO electrode is mechanically weak, and breaks easily causing a problem of increased sheet resistance of the electrode as the electrode substrate for use in a flexible display bends.
Furthermore, in the case of producing a transparent electrode film using a
nanowire that is being studied in various methods, it is important for a transparent electrode to have a high transmittance and high electroconductivity at the same time. These two factors are inversely proportionate to each other, and thus when producing a film with only nanowire, the ratio of transmittance to electroconductivity deteriorates, and thus it is necessary to improve the electroconductivity without undermining the transmittance through thermal treatment.
However, thermal treatment cannot be applied to a flexible high molecular substrate that is weak to heat. In order to improve the electroconductivity through a thermal treatment on a nanowire film, at least 150° C. heat must be applied. But a flexible high molecular substrate would be deformed even under 100° C. of heat, and not only that, the deformation of the substrate deteriorates the electroconductivity of the film.